Coding method for binary digits coding and its circuit for digits transmission

ABSTRACT

This invention presents a coding method for binary digits coding and its circuit for digits transmission. With this coding method, the binary digits are corresponding to a sequence of pulse groups. The binary digits “0” and “1” are corresponding to the two pulse groups with same defined number of pulses and with two special defined pulse frequencies respectively. The said two pulse groups have the same defined number of pulses. The said number is at least 2. The corresponding decoding method divides the said sequence of pulse groups, according to the same defined number, into a set of pulse groups. The duration time of each pulse group is measured. The binary digits “0” and “1” are corresponding to the differences of the duration time of the pulse groups. This invention presents the improvement in data transmission rate and reach in comparing with the relative technology. This invention can be used for data transmission over telephone lines, other electrical cable, or wireless transmission.

FIELD OF THE INVENTION

[0001] The present invention relates the field of digital datacommunication, particularly to the coding/decoding method and thecircuit of using this method for digital data transmission.

BACKGROUND OF THE INVENTION

[0002] ADSL (Asymmetric Digital Subscriber Line) is a technology thatuses the ordinary copper phone line for high-speed Internet connection.ADSL is currently undergoing deployment in North America, parts ofEurope, as well as parts of China.

[0003] The Discrete Multitone (DMT) modulation is the choice for ADSL.As a standard, DMT is adopted internationally by American NationalStandards Institute (ANSI), European Telecommunications StandardsInstitute (ETSI) and the International Telecommunications Union (ITU).There is a complete specification and there are numerous independentmanufacturers developing DMT technology.

[0004] DMT's ANSI T1.413 standard specifies 256 sub-channels, each witha 4 kHz bandwidth. They can be independently modulated from zero to amaximum of 15 bits/sec/Hz. This allows up to 60 kbps per sub-channel. Atlow frequencies, where copper wire attenuation is low and signal tonoise ratio (SNR) is good, it's common to use a very denseconstellation—greater than 10 bits/Hz is typical. In unfavorable lineconditions, modulation can be relaxed to accommodate lower SNR—usually 4bits/Hz or less.

[0005] In July 2002, the ITU completed G.992.3 and G.992.4 as newstandards for ADSL called ADSL2. ADSL2 has been designed to improve therate and reach of ADSL. On long telephone lines ADSL2 provide a datarate increase of 50 kbps. This results in an increase in reach of about600 feet (with original data rate). At the loop reach as 20 kilo feet(about 6 km), the data rate increases from original ADSL's 100 kbps tothe new ADSL2's 150 kbps.

SUMMARY OF THE INVENTION

[0006] The purpose of the present invention is to improve the digitalsignal transmission rate and reach, and a new digital signal codingmethod and its circuit for digital signal transmission is provided.

[0007] For this purpose, the present invention provides a binary digitalsignal coding method that the entire binary digital signal arerepresented by a sequence of pulse group. In this method, the binarydigits “0” and “1” are corresponding to two special pre-defined pulsefrequencies respectively. The two pulse groups have the same pre-definednumber of pulses which is at least 2. This coding method may be calledPulse Group Duration coding (PGD).

[0008] The present invention also provides a decoding method. Thesequence of pulse groups is divided into a set of pulse groups,according to the same pre-defined number. The duration time of eachpulse group is measured. The binary digits “0” or “1” is correspondingto a pulse group according to the differences of the duration time ofthe pulse groups. The duration time is the total time of the periods ofall the pulses in the group, or is the sum of the periods of part ofspecially defined pulses in the pulse group.

[0009] Furthermore, the present invention provides a digital signaltransmission method which includes the process of sending the binarysignals from the transmission side to the reception side through themedium.

[0010] At the transmission side, the binary digits are expressed as asequence of pulse groups and the sequence of pulse groups is sent to themedium. The digits “0” and “1” are expressed by the two pulse groupswith two special pulse frequencies and with same pre-defined number ofpulses. The two pulse groups have the same defined number of pulses.

[0011] At the reception side, the sequence of pulse groups is receivedand divided according to the pre-defined number. The duration time ofthe pulse groups in the sequence of pulse groups are measured and theduration time differences of the pulse groups are used to express thebinary digits “0”and “1”.

[0012] The said digital signal transmission procedure is carried outwithin one sub-channel or a plurality of sub-channels which is withinthe whole bandwidth of the medium. The said two special frequencies arelocated in the sub-channel. The said each of the two special frequenciesis located at the each side of the central frequency of the sub-channel.

[0013] Before the normal digital transmission, a synchronous process isperformed for synchronizing the data transmission and reception and forcorrectly dividing the sequence of pulse groups into the said pulsegroups. This procedure is as follows:

[0014] The mark number, a pre-selected multi bytes binary number, issent from the transmission side repeatedly, and the signals are receivedat the signal reception side. If the received number is not the samewith the said mark number, one pulse is canceled before the next time incomparing the received number with the said mark number until the samenumber is received.

[0015] The present invention also provides the implementation circuit ofthe digital signal transmission. The circuit comprises:

[0016] The transmission medium, which is used for the pulse signaltransmission;

[0017] The coding module, which located in transmission side and used toconvert the binary digits into a sequence of pulse groups in which thedigits “0” and “1” are corresponding to the pulse groups consisting ofsame said pre-defined number of pulses and with two said specialpre-defined frequencies; The pulse groups consist of same pre-definednumber of pulses, and the said pre-defined number is at least 2;

[0018] The band filer and amplifier module, which is located in thereception side and used for the signal band filtering and amplifying;

[0019] The synchronous module, which is connected with the band filterand amplifier module and used for synchronizing the signal transmissionand reception, and dividing the said sequence of pulse groups into saidpulse groups;

[0020] The decoding module, which is connected to the synchronous moduleand used for dividing the said sequence of the pulse groups into thepulse groups according to the said defined number; This module is alsoused for measuring the duration time of each pulse group and convertingthe said duration time differences into binary digits “0” or “1”.

[0021] The said coding module comprises:

[0022] The interface, which is used for converting the transmittingdigits into serial signals and sending the binary digits logical levelsto the voltage level transfer circuit;

[0023] The voltage level transfer circuit, which is used fortransferring the binary logical levels into two special voltage levels;

[0024] The voltage/frequency converter, which is used for generating thepulses with said two special frequencies according to the input two saidspecial voltage levels;

[0025] The binary counter, which is used for counting the pulsesgenerated by the voltage/frequency converter, and as the said definednumber of pulses is counted it controls interface to output the nextdigit bit.

[0026] The said de-coding module comprises:

[0027] The binary counter, which is used for counting the pulses in thesaid sequence of pulse groups, as the said defined number of pulses isreached, it controls the pulse group duration time measurement unit tomeasure the pulse group duration time;

[0028] The pulse group duration time measurement unit, which is used formeasuring the duration time of the pulse groups and output “low” or“high” voltage levels according to the difference of the duration timeof the pulse groups for expressing the binary digits “0”

[0029] The interface, which is used for receiving the voltage outputsfrom the pulse group duration time measurement unit, and converting themas the binary logical voltage levels.

[0030] The said filter and amplifier module comprising filters andamplifiers.

[0031] The said synchronous module comprising:

[0032] The pulse canceling circuit, which is used for each timecanceling one pulse;

[0033] The comparator, which is used for comparing the number outputfrom decoding module with a said pre-selected mark number. If it is notthe same, it controls the pulse canceling circuit to cancel one pulsefrom the sequence of the pulse groups, whereas if it is the same, thepulse canceling circuit will not work.

[0034] The said transmission medium includes telephone line, co-axiscable and electromagnetic field.

[0035] The Pulse Group Duration (PGD) coding method has the followingadvantages:

[0036] 1) The average transmission rate of PGD sub-channel is over 120kbps, while the maximum rate of other related method in the world is 60kbps.

[0037] 2) The experiment shows that PGD's sub-channel can work at themaximum rate in a 6 km long telephone, while the other related techniquein the world can work at the maximum data transmission rate only on thelines shorter than 2 km and the signal to noise ratio is good.

[0038] 3) PGD's implementation is quite simple. Compared with the othercomplicated coding method, PGD can get a more reliable digitaltransmission with a lower cost.

[0039] 4) As the refining on PGD technique, we can enhance themeasurement precision of pulse duration time and use more sub-channelsto get the higher total transmission rate.

[0040] 5) The rate of PGD sub-channel is direct proportion to thecentral frequency of the sub-channel. So we can get the highertransmission rate in a transmission medium with higher workingfrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is PGD coding method illustration according to the presentinvention;

[0042]FIG. 2 is an embodiment of the PGD coding method according to thepresent invention;

[0043]FIG. 3 is a typical PGD coding method application circuit blockdiagram according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0044] The following is the detailed description of the preferredembodiment as shown in the FIG.

[0045] This invention presents a coding method with which the bandwidthof the digits transmission medium is divided into many sub-channels. Twospecial frequencies are selected located at the two sides of the centralfrequency of the sub-channel. In the data transmission side, the pulsegroups are generated corresponding to the two special frequencies. Theduration time differences of the pulse groups are used to express thebinary digits “0” and “1”. In this way pulse group duration coding isachieved. Shown as FIG. 1, the central frequency is f0 in sub-channel 1,the frequencies f1 and f2 located at the two sides of the centralfrequency are selected as the two special frequencies. The first pulsegroup 2 corresponding to the frequency f0 has the duration time t0; thesecond pulse group 3 corresponding to the frequency f1 has the durationtime t1. Notice that the first pulse group 2 and the second pulse group3 have the same number of pulses, as the defined number n, and they havedifferent frequencies. The first pulse group 2 and the second pulsegroup 3 have different duration time. The difference in pulse groupduration time can be used to make difference between the two pulsegroups and further more make the difference between the binary digits“0” and “1”. Shown as FIG. 2, a binary digits “1 0011101” iscorresponding to a sequence of pulse groups.

[0046] In the FIG. 1, and FIG. 2, the first pulse group 2 and the secondpulse group 3 have the same pulse number n, here the n equals to 4. Inusing the telephone lines as the medium for digits transmission, thenumber n is selected according to the length of the line and its signalto noise ratio (SNR). As the digits are transmitted on a long line, thenumber n is selected as a large number to ensure the duration timemeasurement of the pulse groups and minimum miscoding. But to increasethe number n will decrease the data transmission rate. As the length ofthe telephone line is about 6 km the number n can be selected as 4. Whenthe line is very short, the number n can be selected as small one. Thiswill increase the data transmission rate.

[0047] In the signal transmission side, the binary digits are coded inthe sub-channels and the coded pulse signals are sent parallel on themedium. In the reception side, the signals in each sub-channel arereceived parallel. The duration time of each pulse group in all thesub-channel is measured for the binary digits decoding. As shown in FIG.2, the received sequence of the pulse groups is divided into pulsegroups, and each pulse group has 4 pulses. In measuring the durationtime of the pulse groups, two values t0 and t1 are obtained. The t0 iscorresponded to the digit “0” and the t1 is corresponded to the digit“1”. The received sequence of pulse groups is decoded as the binarydigits “10011101”.

[0048] The Pulse Group Duration coding method is different from othercarrier modulation method. In carrier modulation method the carrier doesnot take part of the digits coding. Only the modulation waves take partof the coding procedure. For the Pulse Group Duration coding method ithas no carrier or modulation waves. With the Pulse Group Duration codingmethod two frequencies are selected within a sub-channel. Correspondingto the two frequencies the pulse groups are generated to present thebinary digits “0” and “1”.

[0049] For large amount of digits transmission, the opportunity of thedigit “0” is equal to the opportunity of digit “1”. To select n as thenumber of the pulses in a pulse group, f0 and f1 as the frequencies in asub-channel for binary digits coding, fc as the central frequency of thesub-channel, the central frequency fc can be expressed as:

fc=(f0+f1)/2

[0050] The data transmission rate of a sub-channel, Sr (in bit persecond), equals to the central frequency divided by the number n:

Ss=fc/n

[0051] From the equation above, the data transmission rate of asub-channel is proportional to the central frequency of the sub-channeland inverse proportional to the number n. When multi sub-channel isused, the total data transmission rate equals to the sum of data ratesof all the sub-channels.

[0052] The sub-channel bandwidth dividing is determined by the technicalof the receiver in measuring the time difference of the pulse groups. Ifthe receiver circuit has the accuracy of r % in measuring the timedifference of the pulse group, the bandwidth of the sub-channel Bs canbe:

Bs=*r%

[0053] For telephone lines, besides the POTS the bandwidth can bedivided for up load and down load bandwidth. Suppose the receivercircuit has the ability to identify 1% time difference in measuringpulse group duration, the whole bandwidth can be divided as 50sub-channels. The average data rate of the sub-channels is about 120kbps; the total data rate is 6M bps. To increase the accuracy inmeasuring the time differences of the pulse group duration, the moresub-channels can be divided and the higher data rate can be achieved.

[0054] For correct data transmission and receiving it must correctlydivide each pulse group and each byte. At the same time the datatransmission and receiving must be synchronous. It is important for datatransmission and receiving. It is noticed in the Pulse Group Durationcoding method, a pulse group is consist of n pulses, a bit is expressedby a pulse group. The elements of the pulse groups and bits are thepulses. In applying pulse group duration method for data transmission itmust have the same start pulse for each pulse group at both the datatransmission and receiving side. To identify the start pulse a protocolis made as follows:

[0055] Firstly, a binary number P, consisted of m bytes, is selected andsent from the data transmission side. At data receiving side, thereceived data are compared with P. If the received number is not thesame with P, the number P will be sent from transmission side again. Atthis time one pulse should be canceled from the received serial pulsesbefore the received number is compared with P. This procedure will berepeated again and again until the number P is correctly received at thedata receive side. The P might be sent many times. The maximum times ofsending the number P is equal to 8*n*m. After the pulse groups and thebytes are correctly divided from received serial pulses, both the datatransmission and receive sides are ready for normal data transfer. Thetwo interfaces at the both data transmission and receive side areresponsible for the synchronous between the computers and circuits. Theabove protocol can be executed by a special designed hardware or by ahardware works together with the relative software.

[0056] With the method above, a typical application circuit has beendesigned and made to transfer full duplex digital signals over 6kilometers telephone line (26AWG) between two computers. It comprises oftwo down load sub-channels and an up load sub-channel. The centralfrequency of the three sub-channels are 520 kHz, 450 kHz and 350 kHzrespectively. The number n for the pulse group of the three sub-channelsis selected as 4. With this application circuit, digital transmissionover 6 kilometers telephone line is achieved with the total rate 330kbps.

[0057] This application circuit is shown as FIG. 3. In applying themethod in this invention this circuit executes the coding, decoding, anddigital signal transmission. This circuit comprises the coding circuit4, decoding circuit 8, telephone line 5, the filter and amplify module6, the synchronous module 7, and the line driver 9.

[0058] In the FIG. 3, the interface A is an I/O interface controlled bythe counter A. Each time when it accepts a signal from the counter A, itaccepts a byte of digits parallel from the data transmission computer.The digits in this byte are shifted out one by one. In this way thisbyte of digits become serial data output from interface A. The digitspresent as TTL “high” and “low” voltage level.

[0059] In the interface A, the TTL voltage levels are further convertedinto two special voltage levels. These two special levels are used tocontrol the voltage/frequency converter to generate pulses with twospecial frequencies. For example, the TTL logical levels are convertedto two special levels: 4.1 v and 4.0 v used for the voltage/frequencyconverter to generate pulses with the two frequencies 523 kHz and 517kHz.

[0060] In this typical application, one pulse group consists of 4pulses. The binary counter counts the pulses generated fromvoltage/frequency converter. As the fourth pulse is counted, it gives asignal to control the interface A to shift one bit for generating next 4pulses. These 4 pulses are used to constitute a pulse group. Itsduration time is controlled by the state of the bit “0” or “1”. In thisway, the pulse group duration coding is achieved. These pulse groups aresent to a line driver which sends the signals over a telephone line(26AWG), in length of 6 km, for data transmission

[0061] At another end of the telephone line, the signals are received.The reception circuit uses 6^(th) order filer. The bandwidth of thefilter is designed to meet the bandwidth of the sub-channel in the datatransmission side. For example, for the sub-channel with centralfrequency 520 kHz, the bandwidth of the filter at the data receptionside is designed between 523 kHz and 517 kHz. At the same time of bandfilter the signals are amplified. The total amplify rate is about 10000.The amplitude of the output from amplifier is limited in the rangebetween 0 v to 4.0 v.

[0062] The limited amplitude signals are used to drive the related TTLlogical circuit in the reception circuit. The counter B in the receptionside counts the received pulses. Each time as it counts the fourthpulse, it takes the four pulses as a pulse group. The pulse groupduration time measurement unit is controlled to measure the pulse groupduration time.

[0063] In this typical application, the pulse group duration timemeasurement is substituted by measuring the duration time of the secondpulse and third pulse in the pulse group with 4 pulses. An integrator isused here. At the time of the period of second and third pulse, a gateis open for charging the integrator with a constant electric current.Through two period time of charging, the voltage value at the integratoris proportional to the time of the two periods of the two pulses. At thetime of the fourth pulse of this pulse group this voltage is kept andmeasured. And at the first pulse time of the next pulse group theintegrator is discharged. This process is going repeatedly and generatesa serial of voltage levels.

[0064] In this typical application, the pulse group duration timemeasurement is substituted by measuring the duration time of the secondpulse and third pulse in the pulse group with 4 pulses. It has tworeasons to like this. The first reason is to arrange the time forcharging the integrator, measuring the charge on the integrator, anddischarging the integrator; another reason is more important forachieving a reliable measurement to the duration time of the pulsegroups. In the long line signals transmission, the frequency changebetween two pulse groups needs some interim time. The second and thethird pulses are located in the middle of the pulse group that avoidsthe interim time of the frequency change. Reliable measurement of theduration time of the two pulses is achieved, which is proportional tothe whole duration time of the pulse group, and can be used as thesubstitution to the whole duration time of the pulse group.

[0065] The duration time of the pulse group is expressed by the voltagevalue at the capacitor of the integrator. The higher voltage at thecapacitor expresses longer time duration of the pulse group, and thelower voltage at the capacitor expresses shorter time duration of thepulse group. To convert the two voltage values into TTL logical levels,the decoding is achieved. These TTL logical levels are the serialsignals. In this typical application circuit, these serial signals areconverted into 8 bits parallel signals by the interface B, and sent tothe reception computer.

[0066] In the FIG. 3, the synchronous module 7 is used for synchronizethe transmission signals with the reception signals that comprising: thepulse canceling unit and the comparator. The comparator compares thedecoded signals from decoding module 8 with a pre-selected mark number.If they are not the same, the pulse canceling unit will cancel onepulse. If it is the same, the pulse canceling unit will not work. Forexample, A 4 bytes said mark number P is selected. Send the P from datatransmission side repeatedly. In the comparator, the received 4 bytesnumber is compared with P. The pulse canceling unit will cancel onepulse from received sequence of the pulse groups, and the comparatorstarts the next 4 bytes number compare. These compare process is goingrepeatedly until the mark number P is received. In this time thereceived sequence of the pulse groups is divided correctly and both thedata transmission and reception side are ready for normal digital datatransmission.

[0067] In the typical application circuit, two down load sub-channelsand one up load sub-channel are comprised with full duplex digital datatransmission. In both the data transmission and reception sides thecommercial DSL driver and the relative circuits are used for signalsdrive and echo-canceling.

[0068] In the typical application circuit the telephone line is used asthe digital data transmission medium. The method in this invention canbe applied to other mediums. For example, the method in this inventioncan be used for digital signal transmission over coaxial cable or otherelectric cable. The method in this invention also can be used forwireless digital signal transmission, that medium is the electromagneticwave.

What is claimed is:
 1. A binary digits coding method, wherein the binarydigits are corresponding to the sequence of pulse groups, and the binarydigit “0” and “1” is corresponding to the two pulse groups with twospecial defined pulse frequencies respectively and have the same definednumber of pulses, said defined number at least two.
 2. A decoding methodcorresponding to the coding method as set forth in claim 1, wherein, thesequence of pulse groups are divided according to the said definednumber; the duration time of pulse groups are measured; and the pulsegroups are corresponding to the binary digits “0” or “1” according tothe different duration time of the pulse groups.
 3. The decoding methodas set forth in claim 2, wherein the said duration time of the pulsegroup is the total time of the period time of the all the pulses in thegroup, or is the sum of the period time of part of specially definedpulses in the pulse group.
 4. A digital signal transmission method forexecuting the digits coding as set forth in claim 1 and the decoding asset forth in claim 2, including the process of sending and transferringthe binary signals from the transmission side to the reception side,wherein, At the transmission side, the binary digits are expressed as asequence of pulse groups; The digits “0” and “1” are expressed by thetwo pulse groups with two special pulse frequencies and with samedefined number of pulses, the said pulse groups have the same definednumber of pulses; Sending the sequence of pulse groups to the medium; Atthe reception side, the said sequence of pulse groups is received anddivided according to the said defined number; The said duration time ofthe pulse groups in the sequence of pulse groups are measured and, theduration time differences of the pulse groups are used to express thebinary digits “0” and “1”.
 5. The digital signal transmission method asset forth in claim 4, wherein the signals are transmitted within one ormultiple sub-channels of the whole bandwidth of the medium; and the saidtwo special frequencies are located in a sub-channel.
 6. The digitalsignal transmission method as set forth in claim 5, wherein each of thesaid two special frequencies of the pulse groups is located at the eachside of the central frequency of the sub-channel.
 7. The digital signaltransmission method as set forth in claim 4, wherein a synchronousprocess is included before the normal digit transmission forsynchronizing the data transmission and reception and correctly dividingthe sequence of pulse groups into the said pulse groups.
 8. The digitalsignal transmission method as set forth in claim 7, wherein the saidsynchronous process is as follows: The mark number, a pre-selected multibytes binary number, is sent from the transmission side repeatedly, andthe signals are received at the signal reception side; if the receivednumber is not the same with the said mark number, one pulse is canceledbefore the next time in comparing the received number with the said marknumber, until the same number is received.
 9. A coding circuit forexecuting the coding method as set forth in claim 1, comprising: acoding module used for convert the binary digits into a sequence ofpulse groups in which the digits “0” and “1” are corresponding to thepulse groups consist of same said defined number of pulses and with twosaid special defined frequencies; the pulse groups consist of samedefined number of pulses; the said defined number is at least
 2. 10. Thecoding circuit as set forth in claim 9, wherein said coding modulecomprising: a interface, for converting the transmitting digits intoserial signals and sending the binary digits logical levels to thevoltage level transfer circuit; a voltage level transfer circuit, fortransferring the binary logical levels into two special voltage levels;a voltage/frequency converter, generating the pulses with said twospecial frequencies according to the input two said special voltagelevels; a binary counter, for counting the pulses generated by thevoltage/frequency converter, and as the said defined number of pulses iscounted it controls interface to output the next digit bit.
 11. Adecoding circuit for executing the decoding method as set forth in claim2 comprising: a decoding module for dividing the said the sequence ofpulse groups into the pulse groups according to the said defined numberand for measuring the duration time of each pulse group and thenconverting the said duration time differences into binary digits “0” or“1”.
 12. The decoding circuit as set forth in claim 11, wherein saiddecoding module comprising: a binary counter, for counting the pulses inthe said sequence of pulse groups, as the said defined number of pulsesis reached, it controls the pulse group duration time measurement unitto measure the pulse group duration time; a pulse group duration timemeasurement unit, for measuring the duration time of the pulse groupsand output “low” or “high” voltage levels according to the difference ofthe duration time of the pulse groups for expressing the binary digits“0” or “1”; a interface, for receiving the voltage outputs from thepulse group duration time measurement unit, and converting them as thebinary logical voltage levels.
 13. A digital signal transmission circuitfor executing the method as set forth in claim 4, comprising: medium forsignal transfer; a coding module at the transmission side, forconverting the binary digits as a sequence of pulse groups, the binarydigits “0” and “1” are corresponding to the pulse groups with twospecial pulse frequencies and with the same defined number of pulses,the pulse groups have the same defined pulses number and the saiddefined pulses number is at least two; a band filer and amplifier modulelocated at the reception side for the signal band filtering andamplifying; a synchronous module, connected with the band filter andamplifier module, for synchronizing the signal transmission andreception, and dividing the said sequence of pulse groups into saidpulse groups; a decoding module, connected with the synchronous modulefor dividing the said sequence of the pulse groups into the pulse groupsaccording to the said defined number and, for measuring the durationtime of each pulse group and then converting the said duration timedifferences into binary digits “0” or “1”.
 14. The digital signaltransmission circuit as set forth in claim 13, wherein said codingmodule comprising: a interface, for converting the transmission signalinto serial signal and sending the serial logical voltage level to thevoltage level transfer circuit; a voltage level transfer circuit, forconverting the serial logical voltage levels into two special voltagelevels corresponding to the said two special frequencies; avoltage/frequency converter, for generating pulses with the said twospecial frequencies according to the input two special voltage levelsand; a binary counter, for counting the pulses output from thevoltage/frequency converter as the said defined number is reached itcontrols the interface to output another bit.
 15. The digital signaltransfer circuit as set forth in claim 13, wherein said filter andamplifier module comprising filters and amplifiers.
 16. The digitalsignal transmission circuit as set forth in claim 13, wherein saidsynchronous module comprising: a pulse canceling circuit for each timecanceling one pulse; a comparator, for comparing the number output fromdecoding module with a said pre-selected mark number, if it is not thesame, it controls the pulse canceling circuit to cancel one pulse fromthe sequence of the pulse groups, whereas if it is the same, the pulsecanceling circuit will not work.
 17. The digital signal transmissioncircuit as set forth in claim 13, wherein said decoding modulecomprising: a binary counter, for counting the pulses in the saidsequence of pulse groups; As the said defined number is reached, itcontrols the pulse group duration time measurement unit to measure theduration time of the pulse groups; a pulse group duration timemeasurement unit, for measuring the pulse group duration time andconverting the pulse group duration time differences into binary digits“0” and “1”.
 18. The digital signal transmission circuit as set forth inclaim 13, wherein said transmission medium is the telephone lines or theelectrical cable.
 19. The digital signal transmission circuit as setforth in claim 13, wherein said transmission medium is electromagneticwave.